As a variety of mobile communication services have recently been popular, more frequency bands need to be supported in a single terminal. 2.5th Generation (2.5G) and 3rd Generation (3G) mobile communication systems deployed around the world use different frequency bands in different regions.
Extensive research has been conducted on a portable terminal that can operate in mobile communication systems having different frequency bands. For example, the portable terminal may operate in low-band systems such as Global System for Mobile Communications 850 (GSM 850) and GSM 900 and in high-band systems such as Digital Cellular System (DCS), Personal Communication Services (PCS), and Universal Mobile Telecommunication System 2100 (UMTS 2100), as well. To implement the multi-band terminal, studies have been conducted on an antenna which can operate in multiple bands.
Antennas used for conventional portable terminals include a monopole antenna, a loop antenna, an Inverted F-Antenna (IFA), and a Planar Inverted F-Antenna (PIFA). However, it is difficult to achieve broadband characteristics with these antennas because of a limited space for installing an antenna in a portable terminal.
For example, when a terminal is to operate in low bands such as GSM 850 and GSM 900, a small size and a broad Fractional Bandwidth (FBW) are required for the terminal. Hence, the required bandwidth is hard to secure simply with use of a single antenna. To avert this problem, an IFA-based or PIFA-based switchable antenna has been proposed, which operates at an intended operating frequency by changing the distance between a shorting pin and a feed point through selection of one of shorting pins and thus controlling the impedance of the antenna.
FIGS. 1 and 2 illustrate a conventional PIFA-based switchable antenna configured so as to operate in different frequency bands. Specifically, FIG. 1 is a perspective view of the conventional PIFA-based switchable antenna and FIG. 2 is a plan view of the conventional PIFA-based switchable antenna.
FIGS. 1 and 2, the conventional PIFA-based switchable antenna is configured to include a plurality of shorting pins 101 such that its resonant frequency is changed by controlling its impedance. Specifically, the impedance of the conventional switchable antenna is controlled by selecting one of the shorting pins 101 through a switch 107 and thus adjusting the distance between the selected shorting pin 101 and a feeding point 103.
FIGS. 3 to 6 illustrate operations of the conventional PIFA-based switchable antenna.
FIGS. 3 and 4 illustrate the off and on states of the switch 107, respectively. FIG. 5 is a graph illustrating reflection coefficients S11 with respect to antenna frequencies in the operations of FIGS. 3 and 4, and FIG. 6 is a Smith chart illustrating impedances with respect to antenna frequencies in the operations of FIGS. 3 and 4.
Referring to FIG. 3, since the switch 107 is off, a shorting pin 201 is not shorted to a ground plane 205. Thus, when power is supplied to the switchable antenna, current flows through a feed point 203. Referring to FIG. 4, the switch 107 switches the shorting pin 201 to the ground plane 205. Thus, when power is supplied to the antenna, current flows through the shorting pin 201. In both cases illustrated in FIGS. 3 and 4, as current flows through different shorting pins, the impedance of the switchable antenna is changed. Consequently, the resonant frequency of the switchable antenna may be changed.
The reflection coefficients and impedances of the switchable antenna in the cases of FIGS. 3 and 4 are illustrated in FIGS. 5 and 6.
Referring to FIG. 5, a dotted line 207 represents the reflection coefficients of the switchable antenna in the case of FIG. 3 and a solid line 209 represents the reflection coefficients of the switchable antenna in the case of FIG. 4. Each curve has two valleys and a frequency corresponding to the minimum reflection coefficient of each valley is an operating frequency of the switchable antenna. For example, on the curve 207, a frequency corresponding to the bottom of the left valley 211 is the low-band operating frequency of the switchable antenna (about 850 MHz) and a frequency corresponding to the bottom of the right valley 213 is the high-band operating frequency of the switchable antenna (about 1760 MHz). The same thing applies to the curve 209. However, it is noted from the curves 207 and 209 that there is little difference between the operating frequencies of the switchable antenna in the cases of FIGS. 3 and 4.
Little difference between the operating frequencies in the two cases is also observed in FIG. 6. Impedance variations with respect to antenna frequencies in the operations of FIGS. 3 and 4 are illustrated on the Smith chart of FIG. 6. Reference numeral 215 denotes the impedance of the switchable antenna in FIG. 3 and reference numeral 217 denotes the impedance of the switchable antenna in FIG. 4. Reference numerals 219 and 221 denote impedance variations in low and high bands, respectively. The Smith chart reveals that there is little difference in the distances from the origin (i.e. locuses) regarding impedance variations. The distance from the origin of the Smith chart means the magnitude of impedance. Therefore, when it is said that there is almost no change in the impedance magnitude, this means that there is almost no change in the resonant frequency of the antenna. This result is attributed to the shunt L matching effect of the shorting pins as impedance matching. Due to the shunt L matching, although the phase of impedance may change greatly, a change in the magnitude of the impedance is relatively small.